We're familiar with a substitute teacher. During an absence another teacher comes in to teach the class. We even have "temps" who function like substitutes in the business world.

He was oppressed and afflicted, yet he did not open his mouth...For he was cut off from the land of the living; for the transgression of my people he was punished.....For he bore the sins of many, and made intercession for the transgressors.
- Isaiah 53:7,8,12 TNIV

But the savior of the world is no substitute teacher, or a temp, but a for-eternity substitute who paid the full price of our own sin. He took what we have coming. And hundreds of years before it happened, Isaiah saw it coming.

And because of this, by the mercy of God we have a substitute, we can also say, "Rejoice, in Christ, we are forgiven." Amen.